testing/libmodbus: fix endian test on s390x

This has been fixed in a folk in 2016:
https://github.com/synexxus/libmodbus/commit/a511768e7fe7ec52d7bae1d9ae04e33f87a59627

Upstream PR:
https://github.com/stephane/libmodbus/pull/465
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passed #472
docker-alpine x86_64
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